Magnesium diboride (MgB2) has a critical temperature (transition temperature) of 39 K, which is higher than the critical temperature of conventional metal superconductors (e.g., niobium titanium (NbTi), niobium-3-tin (Nb3Sn)). Therefore, it is possible to operate by a superconducting magnet using MgB2 without using expensive and rare liquid helium (boiling point: 4.2 K). Also, as compared to an oxide-based superconductor, the superconducting magnet using MgB2 has a characteristic that the magnetic field stability is high when operated under a permanent current mode.
It is necessary to wind wire in a coil shape using MgB2 as a wire, to constitute an MgB2 superconducting magnet. As the requirement specification to a wire, two important things are that it is possible to energize at a high current density even in a high magnetic field, and that critical current (Ic) is uniform over a long length (for example, a length of 1 km or more). An MgB2 superconducting wire is generally prepared by a method of filling a powder of a mixed powder of magnesium (Mg) and boron (B) or an MgB2 powder, further added with a third element (carbon, etc.) thereto, in a metal tube, and subjecting it to wire drawing (powder-in-tube method, hereinafter abbreviated as PIT method). In the case of filling the mixed powder of Mg and B (in-situ method), the drawn wire is heat-treated to produce MgB2. Also, in the case of filling the MgB2 powder (ex-situ method), heat treatment is usually performed for improving binding of particles.
FIG. 1A and FIG. 1B are an example of a schematic cross-sectional view of a conventional MgB2 superconducting wire prepared by PIT method. An MgB2 superconducting wire 11 is constituted by an internal MgB2 core 12 and a metal sheath 13 positioned outside the core 12. The metal sheath 13 is constituted by a barrier layer 14 positioned in the side of the core 12 and a stabilizing layer 15 positioned outside the barrier layer 14. The barrier layer 14 is a layer for preventing a reaction of the stabilizing layer with a filler powder in a heat treatment process, and is a layer of iron (Fe), niobium (Nb) or the like. The stabilizing layer 15 is a layer having high electric conductivity and thermal conductivity, and is a layer of copper (Cu) or the like.
It is necessary for improving a current density (critical current density, Jc) which can energize a wire, that the electric connection degree between MgB2 particles is high, specifically, void and different phase (unreacted product, oxide, etc.) between particles are less. By the in-situ method described above, MgB2 is produced after wire drawing, thus MgB2 particles are likely to bind each other. However, volume shrinkage occurs in the reaction of Mg+2B→MgB2, thus even if the filling rate after wire drawing is high as about 90%, it reduces to 67% after heat treatment.
In the in-situ method, as a method for improving an initial filling rate, mechanical milling of raw material powder is effective. Powders of Mg and B and a metal ball are put in a metal container and rotated at a high speed using a planetary ball mill apparatus, whereby hard B powder is sunk into soft Mg powder. When the mixed powder is filled in a metal tube, and the metal tube is subjected to wire drawing, there is almost no void between powder particles, thus the initial filling rate becomes a value close to 100%. Whereby, the filling rate also improves to 74%.
On the other hand, in the ex-situ method, volume shrinkage does not occur like in the in-situ method, thus high filling rate (about 80%) after wire drawing is maintained, but there is a difficulty that the produced MgB2 particles are unlikely to sinter each other. In order to improve sinterability in the ex-situ method, there is also an intermediate method (Premix method) of adding a mixed powder of Mg and B to an MgB2 powder. Relating to the above technologies, Patent Literature 1 and Patent Literature 2 are known.